Effect of Weather Variables on the Inoculum of Diaporthe amygdali Causal Agent of Twig Canker and Shoot Blight in Almond Orchards.

Diaporthe amygdali is the causal agent of twig canker and shoot blight disease on almond. The main objective of this study was to elucidate the effect of weather variables on the D. amygdali inoculum in almond orchards in Mediterranean conditions. For that purpose, a quantitative PCR (qPCR) assay for the detection and quantification of D. amygdali was developed. This methodology was used to detect and quantify inoculum of D. amygdali in spore traps placed at two almond orchards from different locations in two growing seasons (2019 to 2020 and 2020 to 2021). Weather variables were also recorded. Two-part Hurdle models, which include a qualitative part (Bernoulli), with a binary response, and a quantitative part (Gamma), were used to study the relationships of DNA concentration of D. amygdali on the traps with weather variables. The temperature effect was related to the daily thermal amplitude; narrower thermal ranges increased DNA detection, while wider thermal ranges reduced DNA concentration. Days with average relative humidity higher than 80% had a negative effect on the concentration of D. amygdali DNA. Rainfall had a positive influence on both parts of the model, confirming the contribution of precipitation in the abundance of inoculum. Finally, wind speed positively influenced both parts of the models in both growing seasons. The relationships between weather variables and inoculum of D. amygdali will assist to develop a decision support system to optimize the management of twig canker and shoot blight disease on almond.